Correlative Changes of the Gonadotropin-Releasing Hormone and Gonadotropin-Releasing-Hormone-Associated Peptide Immunoreactivities in the Pituitary Portal Plasma in Female Rats

Abstract

The precursor protein of gonadotropin-releasing hormone (GnRH) contains a 56-amino acid peptide, known as GnRH-associated peptide (GAP), and GnRH. Both of these peptides are localized in the same neurons and coprocessed under varieties of physiological conditions. In the present study, we evaluated whether these two peptides are cosecreted into the pituitary portal blood in female rats under the conditions in which the secretion of hypothalamic GnRH and pituitary luteinizing hormone (LH) are known to be altered. The immunoreactivities of GAP-like peptide (IR-GAP-LI) and GnRH (IR-GnRH) in the portal plasma were 2- to 15-fold higher than those observed in peripheral plasma of female rats. In the pubertal females, the preovulatory LH surge which occurred in the afternoon of the day before vaginal opening (puberty) was found to coincide with surges of IR-GAP-LI and IR-GnRH in the pituitary portal plasma. The surges of IR-GAP-LI and IR-GnRH in portal plasma corresponded with a fall in the preoptic and hypothalamic contents of these peptides. In the adult rats, the levels of IR-GAP-LI and IR-GnRH in portal plasma and LH in peripheral plasma were significantly higher during the afternoon of proestrus than those in the afternoon of diestrus. Ovariectomy increased the portal plasma levels of IR-GAP-LI and GnRH and peripheral plasma levels of LH as compared to the level of these hormones in diestrous females. These results indicate that both GnRH and GAP-LI are cosecreted into pituitary portal blood and that changes in the endocrine environment similarly affect both GnRH and GAP secretion.